The present invention relates to power capacitors having a plurality of capacitive section windings within an enclosure and interconnections between the windings and external terminals.
A power capacitor is intended for long life over a wide range of environmental conditions and circuit operating conditions. A multiplicity of factors influence the performance of a power capacitor and its ability to maintain successful operation over a period of many years under a variety of conditions. All of these factors have to be considered in relation to cost in order to achieve the highest level of performance at the lowest cost. This compels attention to the utilization of materials in the best manner possible in order to achieve the highest performance without undue increase in cost.
Power capacitors are commonly made with a plurality of wound capacitor sections in a common enclosure having terminals extending through the enclosure. Each of the section (has a pair of foil electrodes and a plurality of sheets of solid dielectric material between the electrodes. For a variety of reasons, preference is growing in the power capacitor art for the solid dielectric material to consist only of plastic film material, such as polypropylene, rather than capacitor grade paper, or composites of paper and film, as has been used in the part. Two aspects of importance in relation to this general type of capacitor that are dealt with particularly in the present invention are the manner in which the individual capacitor sections are formed and the manner in which the sections are interconnected with each other and with the terminals of the unit.
"Space factor" in a capacitor is defined as the ratio of the distance between a pair of foil electrodes in a capacitor section to the sum of the thicknesses of the one or more sheets of solid dielectric material between these electrodes. A space factor of unity would exist in the case in which the solid dielectric material totally filled the volume between the electrodes. A space factor of less than unity is, of course, not possible. A number of practical considerations dictate that the space factor of an actual capacitor of the type with which we are concerned here will always be something greater than unity. For example, a space factor of 1.30 indicates the distance between electrodes, X, divided by the sum of the dielectric sheet thicknesses, which, for example in an instance of a two layer dielectric of like thickness for each sheet, may be designated as 2Y, where Y is the individual sheet thickness, is 1.3 or the distance x is 30% greater than the sum of the layer thicknesses. Merely to provide some representative numbers, in accordance with the practice in the art prior to this invention in calculating a space factor, a typical example would be one in which the foil electrodes are spaced a distance X of about 1.82 mils and the solid dielectric material consists of two sheets of polypropylene film, each having an individual thickness Y of about 0.70 mils resulting in a space factor of about 1.30.
The practice in the art is for such power capacitors as described to be substantially completely impregnated with a liquid dielectric fluid. This means that there are intended to be no voids in the structure. As space factor is discussed herein, it is to be understood that is relates to the finished capacitor, as formed and ready to be impregnated. The space factor prior to and after impregnation may be in certain cases different because certain impregnants tend to be absorbed within and cause swelling of film type dielectric materials.
Determination of space factor, for a given set of materials and impregnation conditions, is attained by the winding of a specified number and thickness of sheet materials, electrode foils and dielectric sheets, and with control of the tension during winding, and the final pressed dimension of the flattened stack of sections.
For example, to increase the space factor a certain amount, say from 1.20 to 1.30, with a given set of materials, would normally require a reduction of the number of winding turns in the finished section that will occupy a given space in the finished unit. Such adjustments in design are well within the skill of those accustomed to designing and manufacturing such capacitor sections.
As space factor is discussed in this application, it may be determined by certain known procedures such as unwinding a completed section and counting the number of turns of the sheet materials and by measuring the thicknesses of these sheet materials. The space between foils, in a typical section of the final stacked assembly, is computed by reducing the section thickness in the finished unit by the total buildup of electrode materials as determined by the measurements from the unwound section. The buildup of dielectric sheets is computed from the number of turns of such materials that are between foils and the measured thicknesses of these sheet materials. The space factor is then computed from the ratio of the computed distance between foils and the computed thickness of dielectric materials. The figure obtained, as determined by dimensions in the intermediate portion of a section, is reliable as the average space factor of a given section.
Space factor is limited in its acceptable range as capacitors are presently made. A low space factor may be considered undesirable even though it offers the advantage of achieving a smaller volume for a given amount of capacitance and reactive power rating. The dielectric strength and hence the ability of the capacitor to withstand voltage stress, can be unfavorably reduced with a low space factor. This is of particular significance at edges of the foil electrodes that contact the dielectric sheet material because there may be a concentration of electrical field at such points. A low space factor also tends to impair cold temperature switching capabilities. In some climates, capacitors are subjected to extremely cold temperatures such as about -40.degree. C. Switching at such cold temperatures, particularly frequent switching, can cause breakdown or failure which is understood to be because of contraction of the materials in the capacitor and the formation of voids which are free of either solid dielectric material or liquid impregnant. A general difficulty of lwo space factor capacitor windings, of all-film dielectric, is that impregnation is more difficult. There is less facility for the liquid impregnant to penetrate within the section to form a liquid layer on each surface of the dielectric sheet material.
Generally speaking, a looser winding with a higher space factor alleviates the problems mentioned in the preceding paragraph because of the improved ability to impregnate thoroughly. However, a high space factor can be seriously detrimental when using pressure contacts, referred to as tabs, to make contact to the electrode foils for interconnecting sections with each other and the unit terminals. It has been found that even in instances in which the space factor is sufficiently low so that the tabs are securely held within the section, they are still susceptible to arcing, particularly when high current densities occur during capacitor discharge. This arcing can deteriorate the dielectric to the extent of sometimes shorting out the capacitor unit.
The present invention is a combination of elements which permits the space factor to be relatively high, preferably about 1.30, and provides good performance over the totality of expected conditions including extreme cold temperatures and frequent switching. The capacitor sections of this invention are characterized in comprising foil electrodes with an all-film dielectric in wound sections having a space factor of a relatively high value. Conductive tabs are avoided. Instead an extended foil construction is used whereby the respective foils are offset and extend from respective ends of the section, and connections thereto are made by deposited metal, such as a molten solder layer, over the ends of the section. The deposited metal occupies a substantial area of those section ends, such as at least about 30%, which achieves a reliably performing unit which does not have the drawbacks of poor impregnation or the drawbacks of unfavorable arcing conditions because of the use of conducting tabs. In accordance with this invention, prior difficulties in obtaining a satisfactory space factor for insuring impregnation are avoided because the space factor need not be made low for the purpose of insuring good conduction at conductive tabs.
Certain capacitors have been made in the past with an extended foil construction. Such capacitors are known to have been made with a paper dielectric and also with a paper-film dielectric. In these instances, the reasons for practicing the extended foil technique has been to make capacitors with low inductance, or energy storage for capacitor discharge applications, or for high continuous current capacity such as in low voltage capacitors. These are normally not requirements for power factor correction capacitors which may account for the lack of use of extended foil construction in most power capacitors, including all known all-film units. Further, the apparent impairment of impregnation by use of a soldered edge connection tends to lead away from the use of that technique in all-film units. Here, in accordance, with the present invention, the technique of extended foils and soldered edge connections is practiced in an all-film capacitor having a high space factor that is more optimal than prior all-film capacitors in terms of achieving maximum impregnation while retaining secure conductive connections. It is further possible in this invention to use a structure in which the internal edges of each of the electrodes is formed as a rounded and smooth edge such as by folding the edge material a short distance so that the electrode edge is the outside of the fold. This is a means for reducing the susceptibility of the capacitor to voltage breakdown. When the folded or rolled edge is used, there is a natural tendency for the space factor to be increased over most of the section because of the double thickness of electrode foil material at the edge while there is only a single thickness of foil material elsewhere.